The present invention relates to an interferometric measuring device for measuring the shape of a surface of an object. The present invention includes a radiation source emitting a short-coherent radiation, a beam splitter for forming an object beam directed at the object via an object light path and a reference beam aimed at a reflective reference plane via a reference light path. The present invention also includes an image converter which picks up the radiation reflected back from the surface and the reference plane and brought to interference and sends it to an analyzing device for determining a measurement result pertaining to the surface. For the measurement, the optical length of the object light path may be varied relative to the optical length of the reference light path or scanning of an intermediate image of the surface generated in the object light path may occur.
A conventional interterometric measuring device is described in German Published Patent Application No. 41 08 944 which is based on the measuring principle of white-light interferometry or short-coherence interferometry. According to the measuring principle, a radiation source emits short-coherent radiation which is split by a beam splitter into an object beam which illuminates a measuring object and a reference beam which illuminates a reflective reference plane in the form of a reference mirror. To scan the object surface in the depth direction, the reference mirror is moved in the direction of the optical axis of the reference light path by a piezo control element. When the object light path corresponds to the reference light path, the maximum interference contrast is obtained in the area of the coherence length and is detected by a photoelectric image converter and a downstream analyzing device and is analyzed on the basis of the known deflection position of the reference mirror to determine the contour of the object surface.
Additional such interferometric measuring devices and interferometric measuring methods based on white-light interferometry are described by P. de Groot, L. Deck, xe2x80x9cSurface profiling by analysis of white-light interferograms in the spatial frequency domainxe2x80x9d J. Mod. Opt., Vol. 42, No. 2, 389-401, 1995 and Th. Dresel, G. Hxc3xa4usler, H. Venzke, xe2x80x9cThree-dimensional sensing of rough surfaces by coherence radar,xe2x80x9d Appl. Opt., Vol. 31, No. 7, 919-925, 1992.
A conventional interferometric measuring device based on white-light interferometry is also described in German Patent Application No. 199 48 813 (not published previously) wherein to perform measurements in narrow hollow spaces, lateral resolution is increased by producing an intermediate image in the object light path. German Patent Application No. 100 15 878.1, likewise not published previously, proposes scanning of an intermediate image to increase the depth of focus with a relatively high lateral resolution at the same time.
There may be problems with the conventional interferometric measuring devices and measuring methods if the measurement task requires scanning of several separated surfaces which are several millimeters apart, for example, and/or are oriented at an inclination to one another.
An object of the present invention is to provide an interferometric measuring device with which at least two spatially separated surfaces may be measured with accurate and highly reproducible measurement results.
In addition to producing an image of the one surface, an image may be produced of at least one other surface. This may be achieved in the following manner. At least one additional reference plane which is used for depth scanning may be situated in the reference light path according to the number of additional surfaces for generating different optical lengths in the reference light path. The radiation reflected back by the minimum of one additional surface and the respective additional reference plane and also brought to interference and scanned for the measurement may also be sent to the image converter and may be analyzed in the analyzing device for determining the measurement result.
For example, with superposition optics or optics having a corresponding depth of focus, it may be possible to detect and image simultaneously multiple separated surfaces of the same object or different objects, e.g., a guide bore and a valve seat. Splitting the reference light path into partial reference light paths having optical lengths adapted to the different surfaces to be measured may permit simultaneous scanning or scanning in short intervals and therefore rapid scanning of the interference peaks of the various surfaces, for example. The interfering radiation of the various surfaces may be picked up by the image converter simultaneously or in succession and sent to the analyzing device for deriving measurement results, e.g., the position of the various surfaces relative to one another, the height and parallelism and/or the properties of the various surfaces themselves. This may simplify the handling and design of the measuring device.
Configuration options include a superposition optics being situated in the object light path, permitting generation of an image of the one surface and the minimum of one additional surface. Another option is for the additional surface to be imaged on the image converter either directly or via at least one intermediate image in the object light path.
Two alternative exemplary embodiments of the reference light path include a side-by-side configuration or a series configuration of the reference plane and the minimum of one additional reference plane in the reference light path, the minimum of one upstream reference plane being partially transparent in the case of a series configuration. In the case of a side-by-side configuration, different optical elements may be contained in the different partial reference arms.
Different measurement options are obtained due to the fact that the one surface and the minimum of one additional surface belong to objects positioned simultaneously or in succession, the surface and the minimum of one additional surface being situated at different distances.
According to various exemplary embodiments, the object light path for generating a common intermediate image of the surface and the intermediate image of the additional surface(s) may be formed in a common intermediate image plane in the object light path, and the common intermediate image may be imaged on the image converter either directly or via at least one intermediate image. With at least one intermediate image in the object light path, it may be possible to scan an intermediate image and also to obtain an increased lateral resolution.
In accordance with other exemplary embodiments, the reference light path may be formed in a separate reference arm or in a measurement arm belonging to the object light path.
To measure different surfaces in hard to reach places, an optical system, that is rigid relative to the object, may be situated in the object light path, and the rigid optical system may be followed by an optical system that is movable in the direction of its optical axis.
An exemplary embodiment in which the object light path is designed as an endoscope may be advantageous for narrow cavities and a measurement having a relatively high lateral resolution.
The effort of adapting the measuring device to various measurement functions may be facilitated by arranging the rigid optics as part of the optics producing the intermediate image.
To achieve a robust measurement with respect to relative lateral movement of the object, the rigid optics may produce images toward infinity.
If an image of the reference plane or the additional reference plane is within the depth of focus range of the superposition optics, this may contribute to the accuracy of the measurement. It may be advantageous that the image of the reference plane and/or the additional reference plane lie in the plane of the image of the superposition optics, and in addition, with movement of the mobile optics, the image of the reference plane and/or the additional reference plane may move in synchronization with the image plane of the superposition optics.
An exemplary embodiment of the present invention may also include the rigid optics being the optics producing the intermediate image. For example, the rigid optics may be designed as superposition optics with which at least one intermediate image that is rigid relative to the object is produced and an objective optical system designed as movable optics following behind the rigid intermediate image in the path of the beam being movable in the direction of its optical axis for scanning the intermediate image which is normal to this axis in the depth direction and being designed for imaging same directly on the image converter or via one or more intermediate images. Due to the creation of the rigid intermediate image of the object surface situated in the object light path and of the superposition optics in the object light path, the object surface to be measured may be detected with a relatively high lateral resolution even in narrow channels or boreholes and may be evaluated with regard to the depth structure by using the image converter or the downstream analyzing device. The rigid intermediate image may be scanned with relatively simple measures because only few optical components of the object light path need be moved for the depth scanning, the scanned depth of the rigid intermediate image remaining within the range of the depth of focus of the movable objective optics because, due to the depth scanning (depth scan), the object plane of the moving objective optics may likewise be moved through the rigid intermediate image, and in this manner the interference peaks may be analyzed in the area of the greatest focus. In addition, the rigid intermediate image may be aligned or alignable normal to the direction of movement of the objective optics.
With regard to further details concerning the design of the rigid optics and the movable optics, reference is made to the German Patent Application No. 101 15 524.7-52.
For a short measurement time and an accurate measurement, the relative change in the optical length of the object light path and the different optical lengths of the reference light path may occur in synchronization with one another. Measurement of the different surfaces may take place at the same time or at different times.
Possible designs include the superposition optics being designed as free-segment optics having various imaging elements for the surface and the at least one additional surface being designed as multifocal optics or as optics having a depth of focus of at least the greatest optical path difference of the at least two surfaces. A free-segment optical system having multiple light deflection surfaces and lens elements, for example, may be suitable for registering surfaces oriented at an inclination to one another, which may also yield object light paths of different lengths, e.g., for measuring the thickness, diameter or alignment of reference surfaces. A multifocal optics may be suitable, for example, if surfaces oriented parallel to one another and perpendicular to a main beam path of the object light path are to be observed, e.g., measuring of parallelism, thickness and height, whereas optics having a depth of focus of at least the greatest optical path difference of the at least two surfaces may permit simultaneous detection of surfaces oriented parallel to one another with a corresponding distance between them, e.g., measuring of parallelism, thickness and height.
To achieve accurate measurement results, an optical fiber may be provided for illuminating the object with a planar wave, its output on the object end being situated in a telecentric imaging arrangement of the object light path, or an illumination light path having additional lenses and deflector elements may be formed.
The measurement may be made possible or further facilitated by the feature that the reference light path has optics similar or identical to that of the object light path, making it possible to produce the interference or optimize the interference contrast or compensate for optical effects of the components in the object light path.